The present invention relates to a method for controlling a working machine, said working machine comprising a hydraulic system for controlling a plurality of work functions, including lift and tilt of an implement.
Below, the invention will be described in connection with a work vehicle in the form of a wheel loader. This is a preferred, but by no means limiting application of the invention. The invention can for example also be used for other types of working machines (or work vehicles), such as a backhoe loader, an excavator, or an agricultural machine such as a tractor.
A wheel loader can be utilised for a number of fields of activity, such as lifting and transportation of rock and gravel, transport pallets and logs. In each of these activities, different equipment is used, including implements in the form of a bucket, a fork implement and gripping arms. More particularly, the equipment comprises a load-arm unit, or boom, which is pivotally arranged relative to the frame of the wheel loader. Two hydraulic cylinders are arranged between the frame and the load-arm unit in order to achieve a lifting and lowering movement of the load-arm unit. The implement is pivotally arranged on the load-arm unit. An additional hydraulic cylinder is arranged between the implement and the load-arm unit in order to achieve a tilting movement of the implement.
The hydraulic system comprises a pump adapted to supply the hydraulic cylinders with pressurized hydraulic fluid via a hydraulic circuit, comprising a plurality of control valves.
According to prior art, the hydraulic system is load-sensing. According to a previously known such load-sensing system, the maximum available feed pressure is fixed. The maximum feed pressure is then limited either by the pump or by a valve. Furthermore, the hydraulic system is dimensioned for a predetermined highest maximum pressure requirement. In the previously known hydraulic system for wheel loaders, the lifting power can be perceived as too small when the bucket, in a low position, is pushed into a material pile to break out material. In order to solve this, a larger hydraulic cylinder can be used, which then will require a larger pump in order to handle the cylinder speed. The disadvantage is that this means that the system becomes more costly, that it generates more losses in operation and requires a large installation space.
It is desirable to achieve a method for controlling a working machine which, in a cost efficient way, provides an improved operation, particularly with respect to break-out force, preferably with an unchanged or extended service life.
According to an aspect of the present invention, a method comprises determining a maximum pressure of a hydraulic fluid for performing a certain task individually for at least one of the work functions, and supplying the hydraulic fluid, pressurized at most to the determined maximum pressure, to said work function. In this way, a variable maximum pressure, which is demand-controlled for the function, can be obtained.
The requirement of maximum available feed pressure is different depending on the prevailing operating mode, that is to say the function(s) being used, cylinder position, type of implement, handling, etc.
According to a preferred example, the method therefore comprises the step of determining the maximum pressure of the hydraulic fluid individually for the work function based upon the prevailing operating mode. For example, a higher pressure to the lift function can be generated temporarily when the bucket, in a low position, is pushed into a material pile to break out material. Accordingly, the lift cylinder requires a high pressure when it is retracted (penetration into the pile) and a lower pressure when it is extended, which is good from a strength point of view, since cylinders are most sensitive in the extended position.
According to one example, the method comprises the step of continuously determining whether a maximum pressure only at a level below a basic level for the maximum pressure is required to the function and lowering the level of the maximum pressure to the level below the basic level if only the lower maximum pressure level is required. In this way, the lowest possible maximum pressure can be maintained in as many operating modes as possible and thus a long service life can be obtained.
According to one example, the method comprises the step of detecting at least one operating parameter and determining the maximum pressure of the hydraulic fluid individually for the work function based upon the value of the detected operating parameter. The operating parameter then comprises, for example, an operating parameter which is indicative of cylinder position, type of implement, handling being performed, etc. According to one example, the system is adaptive. The control unit can then register how the wheel loader is operated during a certain period of time by detecting operating parameters and make conclusions concerning the handling being performed and/or the type of implement being used. Based thereupon, the control unit can then select a maximum pressure. Alternatively, or as a supplement, the maximum pressure is determined based upon a signal from an operator-controlled element, such as a lever, button or other control means in the cab.
According to another preferred example, the method comprises the steps of determining a maximum pressure of a hydraulic fluid for performing a certain task with the implement individually for at least two of the work functions and delivering the hydraulic fluid, pressurized at most to the determined maximum pressure, to each of said work functions. These work functions include, for example, lift and tilt. The method preferably further comprises the step of supplying the hydraulic fluid, pressurized at most to the determined pressure, simultaneously to each of said work functions.
The hydraulic system is preferably load-sensing. This means that the pump senses the pressure (a LS-signal) from the activated hydraulic cylinders during operation of the system. The pressure signal then originates from pressure sensors which are operatively connected to the hydraulic cylinders. Thereafter, the pump sets a pressure which is a certain number of bar higher than the pressure of the cylinders. This brings about an oil flow out to the hydraulic cylinders, the level of which depends on the extent to which the activated control valve is operated. According to a preferred example, the LS signal is limited depending on the above-mentioned parameters. Only in the case when cooperation between the functions takes place, the valves can limit the maximum pressure in accordance with the above description if a function requires higher pressure. The advantage with limitation primarily by electrical LS is that the losses become lower, since the control pressure for e.g. the lift function is reduced when the lift function is simultaneously stalled.
Further preferred embodiments of the invention and advantages associated therewith are apparent from the following description.